Countermeasures to both visible and thermal sensors, such as camouflage and target signature reduction, have been of great importance in the modern battlefield. One typical infrared (IR) camouflage technique involves covering an object with a material cover, with or without environmentally controlled subsystems (having added extraordinary power requirements). Much effort has been expended in the determination of the materials to be used to comprise the typical IR camouflage covering. One example in the IR camouflage prior art may be found in U.S. Pat. No. 4,156,033 to Earl F. Bienz issued on 22 May 1979. In the Bienz patent there is disclosed applying appropriate foam layers to a tank, so as to mask its thermal signature. In the visible and also in the infrared, these approaches have had limited success.
In the infrared, some factors for the aforementioned limited success includes the following:    a. camouflage material has different heat transfer characteristics from the background resulting in changing apparent temperature differences between the target and the background over a diurnal cycle,    b. camouflage net material is vented to prevent heat build up but winds cause the material to move which effects a blinking IR beacon,    c. one observer seeing an object against a hot background (such as the ground) and a second observer seeing the same object against a cold background (such as the sky), allows for a situation where the current state of the art does not permit the object to simultaneously be made to appear hotter to the first observer and colder to the second observer, and    d. when either the surface and/or the observer moves, the apparent temperature and spatial pattern of the background against which the surface is seen appears to change, thus clearly showing a target.
In the visible, limited success has also resulted from factors including the following:    a. camouflage patterns painted on a conventional surface are unable to change and a fixed camouflage pattern is inappropriate for the variety of backgrounds encountered in nature,    b. one observer sees a military target against a rocky background while another observer sees the target against a forested background while a third observer sees the target against a red barn. The current state of the art does not allow the military target to be effectively camouflaged for all these observers in real time, and    c. when either the military target or the observer move, the background against which the target is seen changes reducing the effectiveness of the camouflage pattern.
Modeling camouflage effectiveness is an area undergoing rapid development, where current modeling methodologies consider both the infrared and visible spectrum. Finding targets in the infrared requires target size and apparent temperature difference between the target and the background, a summary measure that combines target background physical temperature difference and target-background emissivity difference. As current IR sensor technology (of which the sensors are commonly called forward-looking infrared (FLIR) sensors) matures, sensors which respond to apparent temperature difference will be replaced by multi-band radiometers and hyper-spectral line scanners capable of generating contrasts which can separate these two effects; i.e. they can generate a contrast based on the physical temperature differences between the target and the background and a separate contrast based on emissivity differences between the target and the background. Making targets hard to find in the visible is primarily concerned with the development of ever more effective camouflage patterns and with techniques for characterizing the effectiveness of the camouflage for particular terrain.
While the prior art has reported using surface modification devices and techniques, none have established a basis for a specific apparatus and technique dedicated to the task of resolving the particular problem at hand. What is needed in this instance is a real-time control of: 1) the effective emissivity (band averaged or spectral) in the thermal wavelength region, 2) apparent color in the visible wavelength region, and 3) camouflage patterns for both thermal and visible wavelength regions.
The problem is that emissivity is a microscopic property of surfaces that can be controlled by painting but it is difficult to imagine an approach for controlling it in fractions of a second. Those skilled in the art know that a thermal imager responds to the area weighted average emissivity averaged over the detector footprint. For a target at tactical ranges, that footprint has spatial dimensions that are typically between 10 and 100 cm on a side depending on detector size, focal length and target range. The detector footprint is the area on the target seen by a single detector. (For a distant target at range R viewed with a sensor with focal length f the dimension of the footprint Dft is related the detector dimension Ddet, defined as the square root of its area by Dft=(R/f)Ddet)
Accordingly, there is a need in the art to have variable emittance surfaces. The present invention addresses this need.